kyou ha ie ni iru.

は marks 今日 as the topic: as for today or today, ...

So 今日は家にいる。 is structured like:

• 今日は = today / as for today
• 家にいる = be at home

This does not mean today is the grammatical subject. It sets the time frame or topic for the rest of the sentence.

Japanese often leaves out subjects when they are clear from context.

So 今日は家にいる。 can mean:

• I’m staying home today.
• We’re staying home today.
• He/She is at home today.

In many situations, the listener can tell who is being talked about, so Japanese does not need to say it explicitly.

If you want to make it explicit, you could say:

• 私は今日は家にいる。 = I’m at home today.

But very often that would be unnecessary.

に marks the place where someone or something exists / is located with verbs like いる and ある.

So:

• 家にいる = to be at home
• literally: exist at home

This is different from で, which marks the place where an action happens.

Compare:

• 家にいる = be at home
• 家で勉強する = study at home

Since いる describes existence/location, に is the normal particle here.

Japanese uses different existence verbs depending on whether something is animate or inanimate:

• いる = for people and animals
• ある = for things

Since the omitted subject here is a person, you use いる.

Examples:

• 私は家にいる。 = I am at home.
• 本は家にある。 = The book is at home.

Here, 家 usually means home.

Even though 家 can literally mean house, in sentences like this it often refers to the place where someone lives, so English usually translates it as home.

So:

• 家にいる is more natural as be at home not usually be in the house, unless that contrast specifically matters.

Normally, no. 家でいる sounds unnatural in standard Japanese.

That is because いる is a location/existence verb, and with it Japanese normally uses に, not で.

So use:

• 家にいる。 = correct

not:

• 家でいる。 = generally unnatural

A good rule of thumb:

• に with いる / ある
• で with action verbs like 食べる, 勉強する, 働く

It means both, in a way.

Literal grammar:

• 今日 = today
• は = topic marker

So the feeling is as for today.

In natural English, you usually just translate it as today:

• 今日は家にいる。 = I’m staying home today.

The は adds a contrastive or topical nuance depending on context. For example, it can imply:

• Today I’m at home, but maybe not on other days.

It can suggest either, depending on context.

家にいる literally means to be at home, but with 今日 it often sounds like:

• I’ll be at home today
• I’m staying home today

So in real use, it may refer to:

• your current location today
• your plan for the day
• your schedule/status for today

Japanese often leaves that slightly open unless more context is given.

Yes, it is completely natural and complete in Japanese.

Japanese often omits anything that is understood from context, such as:

• the subject
• extra explanations
• pronouns

So even though English might need something like I’m at home today, Japanese can simply say:

• 今日は家にいる。

That is a full, natural sentence.

Some parts can move, but the original order is very natural.

For example:

• 今日は家にいる。 = very natural
• 家に今日はいる。 = possible, but marked/emphatic
• 私は今日家にいる。 = also natural

Japanese word order is more flexible than English, but particles show each word’s role. Even so, some orders sound more neutral than others, and 今日は家にいる。 is a very standard, natural one.

The polite form is:

• 今日は家にいます。

Here, いる becomes います.

So:

• いる = plain form
• います = polite form

Use います in polite conversation, with strangers, teachers, coworkers, etc. Use いる in casual speech with friends, family, or in informal writing.